lazy.rb

lazy.rb provides lazy evaluation and futures in Ruby.

lazy evaluation...

For lazy evaluation, the facilities are similar to those provided by R5 Scheme. There are two functions: promise (similar to Scheme's delay) which takes a block for later evaluation, and demand (similar to Scheme's force), which forces its evaluation (if necessary) and returns its cached result.

Unlike some Scheme implementations, it is safe to pass ordinary values to demand. Promises are also transparent, meaning that in most cases an evaluated promise is not distinguishable from the actual result object it wraps.

To avoid locking overhead for single-threaded applications, promises will not
be threadsafe unless you require 'lazy/threadsafe'.

futures...

Additionally, the library provides futures, where a computation is run
immediately in a background thread. The IO language has these, and they're pretty neat. Futures can be constructed using
future. Based on promises, they are also transparent. An attempt to
demand the result of a future will block until the background thread completes.